The 1989 Gordon Research Conference on the Biology of Aging will be devoted to the topic of the interrelationships between development and aging. It will bring together investigators working on the same biological systems from the developmental and aging aspects. A dialogue will be developed in which we shall elaborate and evaluate the possibility that early developmental events, cell differentiation and high degree of cell and tissue specializations, exert a later (pleiotropic) effect on cellular and organismal viability via loss of adaptability to environmental challenges. Discussions will particularly emphasize the molecular and cellular levels and to a lesser extent the morphological and organismal levels of biological organization. The Conference will open with a session on the state of the art of our understanding, at the molecular and cellular levels, of some basic phenomena in development and aging and conjectures on the possible inter-relationships between these phenomena. The following session will deal with the paradox of the extreme molecular and cellular lens differentiation and yet the very slow rate of age-related functional deterioration of this organ. A session will be devoted to nematode differentiation including studies of cell lineages and programmed cell death and nematode aging. A session on Drosophila differentiation and segmentation and aging will deal with genes involved in development and their possible pleiotropic effects during the post reproductive portion of the life span. The next two sessions will be devoted to developmental and aging aspects of the immune and the hematopoietic systems. These will emphasize the possible role of stem cells and their pool size, the control of their proliferation and differentiation during development and aging. A session on muscle regeneration and the role of extracellular matrix in development, differentiation and aging will ensue. The last session will be devoted to liver development, regenerative capacity and aging at the molecular and cellular levels.